The present invention relates to electric power generating systems, and more particularly, to wind powered generating systems.
For centuries windmills have been used to harness the energy of the wind to grind wheat, operate water pumps and drive various other machines. More recently, wind powered turbines have been connected to generators for producing electric power.
The ever increasing cost of energy has lead to renewed interest in systems which generates electric power from the wind. Such systems are generally reliable, can be used in many locations, are an alternative to expensive fossil fuels, and are non-polluting. These factors make such systems particularly attractive as a partial solution to the energy crisis.
It is desirable for the output of a wind driven electric power generating system to have an AC output with a phase, such as sixty cycles per second, which is compatible with the phase of electric power supplied by utilities. This permits the output of the system to be directly used in the home with appliances, lights and other devices without the use of any DC to AC converters. Also, this enables the output of the wind driven system to be directly connected into the utility power grid. Because of this, most commercial systems for producing electric power from the wind utilize AC or induction type generators. DC generators are less desirable for wind driven systems because their brush structures undergo heavy wear and because electronic DC to AC power conversion circuitry is then required.
In order for an AC generator to provide electric current at a predetermined desired frequency, the input shaft of the generator must be driven at a substantially constant RPM. Therefore, such wind powered generating systems do not operate efficiently over a wide range of wind velocities. This problem can be partially solved by automatically varying the pitch of the blades, by using air brakes on the blades, by using governor mechanisms, and other devices. These devices allow the rotor of the generator to resist over spinning. Increased torque on the blades is translated into higher power output, rather than higher impeller RPM. However, the aforementioned approaches generally waste or ignore a large proportion of the wind energy which could otherwise be translated into electric power. One prior art wind turbine utilizes a small light load generator at low wind velocities and then switches over to a large, heavy load generator at high wind velocities. Both of the generators do not operate simultaneously and considerable efficiency is lost in stepping between a single small generator and a single large generator.